The present invention relates to a liquid-based method for deposition of inorganic films having Cu, Zn, Sn, and at least one of S and Se, and more particularly to a method of deposition of kesterite-type Cu—Zn—Sn—(Se,S) materials and improved photovoltaic devices based on these films.
Large-scale production of photovoltaic devices requires high-throughput technologies and abundant environmentally friendly materials. Thin-film chalcogenide-based solar cells provide a promising pathway to cost parity between photovoltaic and conventional energy sources.
Currently, only Cu(In,Ga)(S,Se)2 and CdTe technologies have reached commercial production and offer over 10 percent power conversion efficiency. These technologies generally employ (i) indium and tellurium, which are relatively rare elements in the earth's crust, or (ii) cadmium, which is a highly toxic heavy metal.
Copper-zinc-tin-chalcogenide kesterites have been investigated as potential alternatives because they are based on abundant elements. However, photovoltaic cells with kesterites, even when produced using high cost vacuum-based methods, have so far achieved at best only <6.7 percent efficiencies, see Katagiri, H. et. al. Development of CZTS-based thin film solar cells; Thin Solid Films 517, 2455-2460 (2009).
K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Solar Energy Mater. Sol. Cells 2009, 93, 583-587 describe a solution-based approach for an indium-free material which produced a photovoltaic device with efficiency of only 1%.
T. Todorov, M. Kita, J. Carda, P. Escribano, Thin Solid Films 2009, 517, 2541-2544 describe a deposition approach based on quaternary Cu—Zn—Sn—S precursors formed by reacting metal acetates and chlorides with elemental sulfur in ethylene glycol at 170° C.
Guo et. al, J. AM. CHEM. SOC. 2009, 131, 11672-11673 have reported films deposited by a similar approach, subsequently subjected to selenization treatment. They have also reported that devices based on the Cu2ZnSnSySe1-y films yield efficiencies of 0.74%, a level that is lower than the above solution approach for Cu2ZnSnS4.
However, there are no reports of hydrazine-based deposition approaches of depositing homogeneous chalcogenide layers from dispersions of metal chalcogenides in systems that are not strictly soluble in hydrazine. Further, there are no reports to extend the nanoparticle- and microparticle-based approaches to systems without organic binders in a manner that particle-based precursors can readily react with solution component and form large-grained films with good electrical characteristics.